The present invention relates generally to a non-destructive structure monitoring sensor and, more particularly, to a Compton backscatter gage apparatus to monitor the structural degradation of composite materials.
The state of the art of backscatter gage apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 4,688,240 issued to Hosemann et al on Aug. 18, 1987;
U.S. Pat. No. 4,582,993 issued to Bhattacharya et al on Apr. 15, 1986;
U.S. Pat. No. 4,277,686 issued to Harding on Jul. 7, 1981;
U.S. Pat. No. 4,258,256 issued to Harding on Mar. 24, 1981; and
U.S. Pat. No. 4,047,029 issued to Allport on Sept. 6, 1977.
The Hosemann et al patent discloses a method for non-destructive testing of structural material. Diffraction scattering and oriented reflection of x-rays at the individual atoms and lattice planes in the crystalline structure are used to determine the integrity of the compound material. X-ray sensitive film or detectors such as scintillation counters are used to determine reflection patterns. Any change in deposition and configuration of the dispersed x-ray over time is indicative of internal tension or stress of the material under test.
The Bhattacharya et al patent discloses a method and apparatus for detecting voids in or on the surface of cast metal. A collimated beam of photons is passed through the cast metal, and the photon flux emerging is detected. The presence of a void in the cast metal will result in an increase in the photon flux received by the photon detector, and a subsequent increase in the output signal.
The Harding '686 patent discloses a device for measuring a scatter coefficient distribution in a plane of a body. The plane is irradiated, and the scattered radiation which is generated is measured by detectors, which enclose the body as completely as possible.
The Harding '256 patent describes a device which detects scattered radiation and reduces the measurement disturbance caused by multiple scatter radiation. Flat laminations are arranged between the primary beam in a fan of flat planes. The flat laminations transmit scattered radiation, which originates in the region of the primary beam, to the detectors, and attenuate scattered radiation, which originates outside of the primary beam.
The Allport patent discloses a gage for determining properties (thickness/density), of sheet material by measuring the attenuation and backscatter of radiant energy, which is directed on the material under test. The gage consists of a pair of radiation detectors. One detector measures the energy attenuation, while the other measures the energy backscatter. Each detector generates electrical signals proportional to the detected energy levels. A computer utilizes this data to determine the desired material properties.
Space structures made of composite materials, and some airborne and earth-bound structures as well, will require constant monitoring to determine the state of their structural health, because of the degradation they suffer in the severe environments in which they operate and the importance of adequate warning of any impending failure.
This monitoring will require non-destructive sensors which can examine critical parts of the structure to determine any sources of incipient failure, such as growing cracks. The subject of health monitoring for composites is new, and has not as yet led to useful techniques, but the possibility of using x-rays for inspection always arises. The conventional concept of bulky x-ray inspection equipment, however, is not adaptable to multiple remote operation in space.
Recently, backscattered x-rays are being exploited as a means of producing one-sided inspection in a variety of applications, frequently, however, these imitate the existing x-ray applications of digital radiography or computer tomography and are even more bulky than simple x-ray techniques. This disclosure presents a concept for using backscattered x-rays on a small scale.
While the above-cited references are instructive, there still remains a need to provide an apparatus which constantly monitors the structural state of the composite materials in severe environments. The present invention is intended to satisfy that need.